1. Field of the Invention
The present invention relates to a moisture adjusting device for adjusting a moisture content contained in a coating film layer formed by applying a coating solution containing an organic solvent, water and the like onto a support, and a planographic printing plate producing method that uses the moisture adjusting device.
2. Description of the Related Art
In recent years, along with development in digital processing techniques for image processing, a photopolymer which is formed so as to have high sensitivity to laser beam in a visible range by using photo-radical polymerization, is often used as a photosensitive material for planographic printing plate in order to implement a direct plate-making system. In the planographic printing plate, provided with a photosensitive layer made of the photopolymer and formed thereon (hereinafter referred to as “photopolymer-type planographic printing plate”), an image forming surface (photosensitive layer) of the planographic printing plate is scanned with a laser beam with a sufficiently small beam diameter based on image data, so that a photo-polymerization reaction occurs at the photosensitive layer provided on a support, thereby curing the exposed portion thereof. Namely, by using the photopolymer-type planographic printing plate, characters and images can be directly formed on the image forming surface of the planographic printing plate without using a film original (lith film). Because oxygen in the air becomes a factor in inhibiting a photo-polymerization reaction, the photopolymer-type planographic printing plate is usually provided with an overcoat layer that serves as an oxygen blocking film, made of a transparent resin such as a polyvinyl alcohol (PVA) and is provided on the surface of the image forming surface. The photosensitive layer is covered with the overcoat layer.
In most cases, a support for such planographic printing plate is produced as follows. Namely, a roughening treatment is performed on one side surface or both side surfaces of an elongated band-shaped aluminum plate made of aluminum or an alloy comprising aluminum as a main component (hereinafter referred to as an “aluminum alloy”). Then, an anodized film is formed by an anodizing treatment in order to improve wear resistance. A photosensitive material (photopolymer) is applied on the anodized film on the support, and dried, whereby a photosensitive layer is formed. Subsequently, a PVA is applied so as to cover the entire surface of the photosensitive material and dried, so that an overcoat layer is formed. A protective interposing paper (hereinafter simply referred to as an “interposing paper”), for example, may be adhered to a product web as a blank of the planographic printing plate produced as described above. Further, the product web may be cut into a product size. In this way, the product web is processed into a planographic printing plate product.
It is known that a product quality of the above-described planographic printing plate is influenced greatly by a moisture content of the overcoat layer. A large moisture content may lead to an increase in sensitivity, and thus fogging problems may easily occur. On the other hand, a small moisture content may lead to a decrease in sensitivity. Accordingly, the planographic printing plate has a problem in that the quality thereof becomes unstable when the moisture content of the overcoat layer is not within an appropriate range.
In recent years, a wide variety of photopolymer-type planographic printing plates with different properties have been developed in accordance with various needs of users. These photopolymer type planographic printing plates have different appropriate moisture contents in their overcoat layers depending on characteristics such as photosensitivity and the like. For this reason, a producing line for planographic printing plate is required to be capable of precisely adjusting the moisture content of the overcoat layer to different target values depending on the type of the photopolymer type planographic printing plate. In order to meet such requirements, the producing line for the photopolymer type planographic printing plate is usually provided with a humidity conditioning zone for adjusting the moisture content of a product web with the overcoat layer being formed thereon. By adjusting a humidity within the humidity conditioning zone, the moisture content of the overcoat layer of the product web which has passed through the humidity conditioning zone can be adjusted to a target value with high precision.
However, if the moisture content of the overcoat layer of the product web is adjusted to a target value with high precision in the humidity conditioning zone, when an interposing paper is later adhered to the surface of the overcoat layer, the moisture content of the overcoat layer be changed over time by the moisture content of the interposing paper. As a result, the moisture content of the overcoat layer may deviate from the target value. Moreover, the speed of the producing line for planographic printing plate has been increasing. Thus, if the moisture content of the overcoat layer before being subjected to the moisture adjustment greatly deviates from a target value, it is difficult to adjust with high precision the moisture content of the product web to a target value, with the overcoat layer being formed thereon, only in the humidity conditioning zone with limited length.